1. Field of the Invention
The present invention relates to an apparatus for detecting a biological object. The apparatus according to the invention is used for a personal identification system in which a person is identified by identifying a fingerprint of the person.
In a personal identification system using fingerprint identification, a fingerprint sensor is provided for reading a fingerprint, as a picture image, of a person, and a processing device for generating reference data from the picture signal read by the fingerprint sensor, registering the generated reference data, and comparing the picture signal read by the fingerprint with the reference data to identify whether or not the fingerprint belongs to an authorized person. In such a personal identification system, the fingerprint sensor must read the fingerprint picture image clearly and correctly without distortion.
In general, in a fingerprint sensor, a detection light is irradiated at a preselected angle onto the ridge line portions and the groove line portions of the fingerprint pressed against a light conducting plate (a transparent plate), through this transparent plate. According to Snell's law, only light reflected from the ridge line portions of fingerprint is transmitted by the full reflection inside thereof through the transparent plate to reach a light receiving element and produce a fingerprint picture electrical signal. The light reflected from the groove line portions of fingerprint is not transmitted by such a full reflection, and thus such an electrical signal is not produced.
In such a fingerprint sensor, a fraudulent operation of the fingerprint identification system having such a biological object detection means, with criminal intention, can be carried out by using a replica of a human finger, made of rubber or plaster, and therefore, protection from fraudulent use of the personal identification system by a false identification of a fingerprint by the fingerprint sensor was required.
A number of attempts have been made to provide such protection against a fraudulent operation of the fingerprint sensor. For example, a pulsating electrical signal corresponding to the pulsation of the blood flow in the human finger is derived from a light receiver receiving light irradiated from a light emitter and transmitted through the finger to identify the fingerprint thereof.
In another example, an electric current corresponding to the skin resistance of a human finger is measured by an electrical circuit formed by a pair of electrically conductive electrodes against which the human finger is pressed. In a further example, a change with time of the degree of contact between the surface of the human fingerpad (the palm side of a human fingertip) and the surface of the transparent plate for the fingerprint detection, which is affected by perspiration from the surface of the human fingerpad, is detected by an electrical signal output from the light receiver. Nevertheless, these attempts at protection against fraudulent operation were not successful, because a considerable length of time is required for the detection or a very precise detection cannot be obtained, and thus practical use of the above attempts was not adequate.
A prior art fingerprint sensor having a biological object discriminating means has been proposed, in which the nature of the skin of a living human fingerpad, i.e., that the spectral reflectance of the skin of a living human fingerpad to which a pressure is applied is different from that of a non-living object such as a replica finger, is utilized.
The color of the skin of a finger not under pressure is usually reddish, but becomes whitish when a pressure is applied to the skin of the finger by, for example, pressing the fingerpad against a plate. It has been acknowledged that the spectral reflectance of the light in the red spectral range, i.e., the light wavelength of approx. 640 to 770 nm, does not show a substantial difference between the pressed state and the not pressed state, and the spectral reflectance of the light in the blue and green spectral range, i.e., the light wavelength of approx. 400 to 600 nm, in the not pressed state is much less than in the pressed state. Accordingly, by measuring the spectral reflectance in the blue and green spectral range of the surface of an object in question, it is possible to detect whether or not this object is a biological object.
In this prior art biological object detection means there is provided a finger nipping member constituted by a pair of nip elements for nipping the tip of a finger therebetween, and a spring bridging these nip elements for developing a force to cause these nip elements to be biased toward each other. Each nip element is provided with a light emitting element for emitting a light having a spectral wavelength range including the blue or green range, and a light sensing element for responding to a light having the spectral wavelength range of blue or green.
Namely, when a fingertip is forced into the gap between these nip elements against the force of the spring thereof, the gap between these nip element is enlarged and thus pressure is applied to the sides of the fingertip by the action of the spring. As the pressure applied to the sides of the fingertip increases, the color of the skin of the finger changes from reddish to whitish, to thus change the spectral reflectance, and accordingly, the value of the light having the spectral wavelength of the blue and green ranges detected by the light sensing element is increased. Therefore, the biological object detection is carried out by using the result of the detection of the reflected blue or green range light by the light sensing element.
Nevertheless, a fraudulent operation of the fingerprint identification system having such a biological object detection means can be carried out by lo using a first replica of a finger for counterfeiting a human fingerprint and the spectral reflectance characteristic of human finger skin in the pressed state and a second replica of a finger for counterfeiting the spectral reflectance characteristic of human finger skin in the not-pressed state. The fraudulent operation is carried out by, first forcing the first replica of a finger covered by the second replica of a finger into the space between the above-described nip elements to imitate the human finger in the not-pressed state, and second, taking the second replica of a finger out of the first replica of a finger to imitate the human finger in the pressed state. Accordingly, protection of the personal identification system by fingerprint identification against such fraudulent operation of the fingerprint sensor is urgently required.